Solar collectors are known in the art. Example solar collectors are disclosed in U.S. Pat. Nos. 5,347,402, 4,056,313, 4,117,682, 4,608,964, 4,059,094, 4,161,942, 5,275,149, 5,195,503 and 4,237,864, the disclosures of which are hereby incorporated herein by reference. Solar collectors include at least one mirror (e.g., parabolic or other type of mirror) that reflects incident light (e.g., sunlight) to a focal location such as a focal point. In certain example instances, a solar collector includes one or more mirrors that reflect incident sunlight and focus the light at a common location. For instance, a liquid to be heated may be positioned at the focal point of the mirror(s) so that the reflected sunlight heats the liquid (e.g., water, oil, or any other suitable liquid) and energy can be collected from the heat or steam generated by the liquid.
FIG. 1 is a schematic diagram of a conventional solar collector, or a part thereof, where a parabolic mirror 1 reflects incident light (or radiation) from the sun 3 and focuses the reflected light on a black body 5 that absorbs the energy of the sun's rays and is adapted to transfer that energy to other apparatus (not shown). By way of example only, the black body 5 may be a conduit through which a liquid or air flows where the liquid or air absorbs the heat for transfer to another apparatus. As another example, the black body 5 may be liquid itself to be heated, or may include one or more solar cells in certain example instances.
FIG. 2 is a cross sectional view of a typical mirror used in conventional solar collector systems. The mirror of FIG. 2 includes a reflective coating 7 supported by a single bent glass substrate 9, where the glass substrate 9 is on the light incident side of the reflective coating 7 (i.e., the incident light from the sun must pass through the glass before reaching the reflective coating). This type of mirror is a second or back surface mirror. Incoming light passes through the single glass substrate 9 before being reflected by the coating 7; the glass substrate 9 is typically from about 4-5 mm thick. Thus, reflected light passes through the glass substrate twice in back surface mirrors; once before being reflected and again after being reflected on its way to a viewer. Second or back surface mirrors, as shown in FIG. 2, are used so that the glass 9 can protect the reflective coating 7 from the elements in the external or ambient atmosphere in which the mirror is located (e.g., from rain, scratching, acid rain, wind-blown particles, and so forth).
Conventional reflectors such as that shown in FIG. 2 are typically made as follows. The single glass substrate 9 is from about 4-5 mm thick, and is heat-bent using temperatures of at least about 580 degrees C. The glass substrate 9 is typically heat/hot bent on a parabolic mold using such high temperatures, and the extremely high temperatures cause the glass to sag into shape on the parabolic mold. After the hot bent glass is permitted to cool to about room temperature, a reflective coating (e.g., silver based reflective coating) is formed on the bent glass substrate. Ceramic pads may then be glued to the panel which may be bolted to a holding structure of the solar collector.
Unfortunately, the aforesaid process of manufacturing reflectors is problematic for at least the following reasons. First, reflectance of the product shown in FIGS. 1-2 is less than desirable, and could be subject to improvement (i.e., it would be desirable to increase the reflectance). Second, during the manufacturing process, it is necessary to mirror-coat a 4-5 mm thick pre-bent glass sheet (a 4-5 mm thick pre-bent glass sheet will not sag flat during the mirror-coating process), and applying such coatings to bent glass is difficult at best and often leads to reduced reflective/mirror quality.
Thus, it will be appreciated that there exists a need in the art for a more efficient technique for making bent reflective coated articles, and/or for a more efficient mirror for use in solar collectors or the like. An example of such an article is a mirror which may be used in solar collector applications or the like.
In certain example embodiments of this invention, a parabolic trough or dish reflector/mirror laminate for use in a concentrating solar power apparatus is made by: (a) forming a reflective coating on a thin substantially flat glass substrate (the thin glass substrate may or may not be pre-bent prior to the coating being applied thereto; if the thin glass substrate is pre-bent prior to application of the coating thereon then its thin nature and large size/weight will permit the glass to sag so as to be flat or substantially flat in the coating apparatus when the coating is applied thereto, such that the coating is still applied to a flat or substantially flat glass substrate even though it may have been pre-bent), (b) optionally, if the thin glass substrate in (a) was not pre-bent, cold-bending the thin glass substrate with the reflective coating thereon; and (c) applying a plate or frame member to the thin bent glass substrate with the coating thereon from (a) and/or (b), the plate or frame member (which may be another thicker pre-bent glass sheet, for example) for maintaining the thin glass substrate having the coating thereon in a bent orientation in a final product. It is noted that (b) and (c) may be performed at the same time, or in entirely different steps, in different example embodiments of this invention. For example, the thin glass substrate with the coating thereon may be cold-bent when it is pressed against the plate or frame member during the laminating process, so that (b) and (c) would be performed right after one another or at essentially the same time. Alternatively, the thin glass substrate with the reflective coating thereon may be cold-bent and after the cold bending could be brought to and coupled with the plate or frame member. The reflective coating may be a single layer coating, or a multi-layer coating, in different example embodiments of this invention.
In certain example embodiments, the mirror/reflector laminate is a parabolic dish or trough type reflector and reflects incident sunlight (e.g., visible and/or IR radiation) and focuses the same at a common location. For instance, a liquid to be heated may be positioned at the focal point of the parabolic mirror(s) so that the reflected sunlight heats the liquid (e.g., water, oil, or any other suitable liquid) and energy can be collected from the heat or steam generated by the liquid.
In certain example embodiments of this invention, when the thin glass substrate is not pre-bent prior to forming the reflective coating thereon, the thin glass substrate with the reflective coating thereon may in (b) be cold-bent at a temperature of no more than about 200 degrees C., more preferably no more than about 150 degrees C., more preferably no more than about 100 degrees C., even more preferably no more than about 75 degrees C., still more preferably no more than about 50 degrees C., still more preferably no more than about 40 or 30 degrees C. The cold-bent thin glass substrate with the reflective coating thereon may then be laminated to the plate or frame member (which may be another thicker pre-bent glass sheet, for example) for maintaining the coated glass substrate in a bent orientation in a final product.
In certain example embodiments, the plate or frame member may be flat and may be applied to the thin glass substrate prior to bending thereof. Then, the plate member (e.g., of glass, thermoplastic, or the like) and the thin glass substrate can be bent together with the plate or frame member optionally being pre-heated to permit more efficient bending thereof. In certain example embodiments of this invention, the plate or frame member may be another glass substrate/sheet that is thicker than the thin glass substrate having the reflective coating thereon, and may optionally have been pre-bent (e.g., via hot bending) prior to being laminated to the thin glass substrate and/or reflective coating. The pre-bent (via hot-bending) thick glass substrate/sheet may be laminated/adhered to the thin glass substrate with the reflective coating thereon via an adhesive/laminating layer which is typically polymer based (e.g., PVB, or any other suitable polymer inclusive adhesive).
In certain example embodiments, the reflective coating may be designed so as to better adhere to a polymer based adhesive/laminating layer that is used to couple the plate member (e.g., glass sheet) to the thin glass substrate. For example, in certain example embodiments, the reflective coating is a mirror coating and includes a passivating film comprising copper, tin oxide, and/or silane(s), optionally with paint thereon, for good adhering to the polymer based adhesive/laminating layer which may be made of polyvinyl butyral (PVB) or the like.
In certain example embodiments of this invention, there is provided a method of making a mirror for use in a concentrating solar power apparatus, the method comprising: bending a thick glass substrate having a thickness of at least 2.0 mm into a desired bent shape so as to form a pre-bent thick glass substrate; forming a mirror coating on a thin glass substrate having a thickness of from about 1.0 to 2.0 mm, the mirror coating being formed on the thin glass substrate when the thin glass substrate is in a substantially flat shape; and after the mirror coating has been formed on the thin glass substrate, laminating the thin glass substrate to the pre-bent thick glass substrate using at least one polymer inclusive adhesive layer to form a laminate mirror comprising a substantially parabolic shape, wherein the laminate mirror is used in a concentrating solar power apparatus and has a solar reflectance of at least 90%.
In certain other example embodiments of this invention, there is provided a method of making a mirror for use in a concentrating solar power apparatus, the method comprising: bending a thick glass substrate into a desired bent shape so as to form a pre-bent thick glass substrate; forming a mirror coating on a thin glass substrate, the mirror coating being formed on the thin glass substrate when the thin glass substrate is in a substantially flat shape; wherein the thin glass substrate has a thickness smaller than that of the thick glass substrate; and after the mirror coating has been formed on the thin glass substrate, laminating the thin glass substrate to the pre-bent thick glass substrate using at least one polymer inclusive adhesive layer to form a laminate mirror to be used in a concentrating solar power apparatus.
In other example embodiments of this invention, there is provided a concentrating solar power apparatus including at least one mirror, the concentrating solar power apparatus comprising: a bent laminate mirror comprising a thick glass substrate having a thickness of at least 2.0 mm, a thin glass substrate having a thickness of from about 1.0 to 2.25 or 1.0 to 2.0 mm, and a mirror coating formed on the thin glass substrate, the thin glass substrate being laminated to the thick glass substrate with at least one adhesive layer so that the adhesive layer and the mirror coating are both located between the thin and thick glass substrates; and wherein the bent laminate mirror is substantially parabolic in shape and has a solar reflectance of at least 90%.
In certain cases, flat, parabolic, spherical, or otherwise shaped and/or arranged laminated or monolithic mirror panels for use in solar concentrating systems would benefit from additional stiffness. For example, an increase in stiffness would help to meet high wind, dimensional stability, and/or other requirements. This is true not only for hurricane-prone areas, but also areas that experience moderate winds that could be strong enough to cause a mirror to avoid holding a tight focus. In general, laminated or monolithic mirror panels for use in solar concentrating systems will deflect at least some wind but also will vibrate because of such winds. Indeed, vibration and deflection during operation results in some de-focusing of the system, with the system being extremely sensitive to small changes or error in panel shape, e.g., whether that shape is parabolic or otherwise. At a first level of interference caused by wind, the laminated or monolithic mirror panels will not perform at peak efficiency and/or will lose energy, since the mirrors will not be able to accurately focus light in the appropriate area. For example, a mirror having a diameter of about 8 meters may not be able to adequately focus the light on a hole or aperture of only a few inches in diameter. At second level of interference, a mirror will fail completely and may even become damaged in the process.
Simply adding glass thickness will help to increase rigidity. However, adding glass thickness quickly results in large increases in panel mass which, in turn, drives a need for stronger, more expensive support structures. Furthermore, another impeding factor for thicker monolithic glass is the increased transmission path of light to the mirror surface and the resulting drop in reflectivity of the mirror and hence efficiency of the energy collection. Fractions of percentage points of reflectivity are competitive drivers in these panels; thus, adversely affecting reflectance can have a disadvantageous impact on the assembled products. Therefore, simply adding glass thickness may not always be a viable, cost-effective option.
Another option involves bonding a whole separate structure to substantially all of the monolithic or glass mirror. However, this technique also becomes expensive. In addition, it is difficult to bond materials to glass on a substantially permanent basis. Indeed, such structures likely would not meet durability requirements, which typically require survivability throughout a 10-30 year period in a desert climate. Additionally, the different materials likely will have different coefficients of thermal expansion (CTE). Because the two different materials (e.g., the glass and the material bonded to it for support) will expand and/or contract at different temperatures relative to one another, delamination and/or breakdown of the components will occur. Furthermore, UV penetration oftentimes will hasten such delamination and/or breaking down of the components.
Thus, in addition or in the alternative to the above, it will be appreciated that there is a need in the art for increasing the thickness of mirror panels in solar concentrating systems or the like.
Accordingly, certain example embodiments provide one or more stiffening rib(s) that are preformed to the part shape and are bonded to the back of the glass to increase overall panel stiffness. This arrangement advantageously adds stiffness without unduly increasing weight in certain example embodiments.
In certain example embodiments of this invention, a stiffening rib for a reflector in a solar collector system is provided. At least one area suitable for accommodating a polymer-based adhesive for bonding the rib to a side of the reflector facing away from the sun is provided. The stiffening rib is formed so as to substantially match a contour of the reflector. At least two of the rib, the reflector, and the adhesive have respective coefficients of thermal expansion within about 50% of one another. The stiffening rib is sized and positionable on the reflector so as to increase an EI value of the reflector at least about 10 times or to at least about 9,180 pascal meters4.
In certain example embodiments of this invention, there is provided a solar collector system including a plurality of reflectors, with each said reflector having a stiffening rib associated therewith and attached thereto on a side facing away from the sun. At least one area on each said stiffening rib is suitable for accommodating a polymer-based adhesive for bonding the rib to a side of the associated reflector facing away from the sun. Each said stiffening rib is formed so as to substantially match a contour of the associated reflector. At least two of each said rib, the associated reflector, and the adhesive have respective coefficients of thermal expansion within about 50% of one another. Each said stiffening rib is sized and positioned on the associated reflector so as to increase an EI value thereof at least about 10 times or to at least about 9,180 pascal meters4.
In certain example embodiments of this invention, a method of making a solar collector system including a plurality of reflectors is provided. Each said reflector has a stiffening rib associated therewith. Each said stiffening rib is bonded to the associated reflector via a polymer-based adhesive, with each said stiffening rib being bonded to the associated reflector on a side facing away from the sun. Each said stiffening rib is contoured to substantially match a shape of the associated reflector. At least two of each said rib, the associated reflector, and the adhesive have respective coefficients of thermal expansion within about 50% of one another. Each said stiffening rib is sized and positioned on the associated reflector so as to increase an EI value thereof at least about 10 times or to at least about 9,180 pascal meters4.
In certain example embodiments of this invention, a method of making a stiffening rib for a reflector in a solar collector system is provided. This method comprises roll-forming steel, injection molding plastic or glass-filled plastic, or extruding aluminum so as to form the stiffening rib. The stiffening rib is formed so as to include at least one area suitable for accommodating a polymer-based adhesive for bonding the rib to a side of the reflector facing away from the sun. The stiffening rib is formed so as to substantially match a contour of the reflector. At least two of the rib, the reflector, and the adhesive have respective coefficients of thermal expansion within about 50% of one another. The stiffening rib is sized and positionable on the reflector so as to increase an EI value of the reflector at least about 10 times or to at least about 9,180 pascal meters4.
The features, aspects, advantages, and example embodiments described herein may be combined in any suitable combination or sub-combination to realize yet further embodiments of this invention.